3 Answers
3

The Maharil and the Seder HaYom (from the times of the Arizal) both say that we do not eat meat even for the Seudas Rosh Chodesh. The reason, says the Seder HaYom is that we eat meat on Rosh Chodesh because, as the first day of the new month, it is a day of Simcha. Not so Rosh Chodesh Av which is a month of Puranus, troubles. Also it is the day that Aharon HaKohen was niftar. This is also the opinion of the Arizal in Shaar HaKavanos.

Summary; Askenazim focus on the fact that normally Rosh Chodesh is a simcha but here it is the begining of a bad thing so no Simcha, Plus it is Ahron Yahrtzeit which ussaly calls for a fast.

The reasoning for Sfardim is:

The Shiyarei Knesess HeGedola writes that the minhag in Turkey, at the time, was to eat meat and drink wine during the Rosh Chodesh Seudah, even in Av. This is also the opinion of the Chida. The Toras Chaim says that if it is your minhag to consistently eat a celebratory seudah with meat and wine each Rosh Chodesh, you should do so on Rosh Chodesh Av as well. The Darkei Chaim V’Shalom takes this a step further and says you should only eat meat if you are noheg like the Talmidei Baal Shem Tov and make a public seudah every Rosh Chodesh. (See Minhag Yisroel Torah 551:6)

Summary: Sfardim and it seems Chassidim too celebrate regularly and do not focus on the negative aspect of the month because Rosh Chodesh is still a simcha. I guess it is consistent with them in genral being more in tune with Rosh Chodesh.

Sefardim, following the Mechaber, don't start the mourning period until the week of Tisha B'av, and Rosh Chodesh Av is never in the week of Tisha B'av. Hence they would have no problem with eating meat on Rosh Chodesh Av.

Ashkenazim, following the Rema, start the mourning period on Rosh Chodesh (hence "The Nine Days") and therefore don't eat meat on Rosh Chodesh Av.

This issue depends on whether you are sefardi or ashkenazi. Although a minority opinion holds that if one always has a meaty rosh chodesh seuda (meal) then he may do so on rosh chodesh av as well, the vast majority of poskim — including the chayei adam (133,15) and mishna brura (551, 58) — rule that one may not have meat on rosh chodesh itself; they hold the nine days are just that — nine days. Mishenichnas av — meaning rosh chodesh — memaatin besimcha — this means that as soon as it is rosh chodesh av the restrictions already apply.

Yet, even the sefardi poskim who are machmir on the nine days (the shulchan aruch cites three opinions with no ruling) hold that one may eat meat on rosh chodesh av itself — including the ben ish chai (year 1 devarim 15) and kaf hachaim (551, 125 -126). They hold lo ad bchlal (not including the boundary point), especially as sefardi poskim technically never took on 3 weeks or nine days, rather shavua shechal bo; therefore those sefardim who are machmir and start restrictions from the nine days at least still have rosh chodosh av to be lenient with. I think the lashon (wording) in the kaf hachaim is "v'dye bazeh" (and this suffices).